The SGE category was created by Congress as a way to apply an important, but limited set of conflict of interest requirements to a group of individuals who provide important, but limited, service to the Government.
SGEs provide temporary service to the Government (not to exceed 130 days during any period of 365 consecutive days with or without compensation). SGEs are often recruited because they provide outside expertise or perspectives that might be unavailable among an agency's regular employees. SGEs are generally used as advisory committee members, individual experts or consultants. However, some SGEs serve on Boards or Commissions and are brought on as per applicable statute. (See 18 U.S.C. 202(a) for more information. Link to text of 18 U.S.C. 202)
If an agency designates you as an SGE, but you unexpectedly serve more than 130 days during 365 consecutive days, you'll still be an SGE for the remainder of that period. But, during the next 365-day period, the agency should reevaluate whether you should still be designated as an SGE.